Total Synthesis of Antibiotics GE2270A and GE2270T

Antibacterial Thiopeptide GE2270-Congeners from Nonomuraea jiangxiensis

Thiopeptides are macrocyclic natural products with potent bioactivity. Nine new natural thiopeptides (1−9) were obtained from a Nonomuraea jiangxiensis isolated from a terrestrial soil sample collected in Singapore. Even though some of these compounds were previously synthesized or isolated from engineered strains, herein we report the unprecedented isolation of these thiopeptides from a native Nonomuraea jiangxiensis. A comparison with the literature and a detailed analysis of the NMR and HRMS of compounds 1−9 was conducted to assign their chemical structures. The structures of all new compounds were highly related to the thiopeptide antibiotics GE2270, with variations in the substituents on the thiazole and amino acid moieties. Thiopeptides 1−9 exhibited a potent antimicrobial activity against the Gram-positive bacteria, Staphylococcus aureus with MIC90 values ranging from 2 µM to 11 µM. In addition, all compounds were investigated for their cytotoxicity against the human cancer cell line A549, none of the compounds were cytotoxic.

Palladium-catalyzed direct C2-arylation of azoles with aromatic triazenes

A highly efficient palladium-catalyzed arylation of azoles at the C2-position using 1-aryltriazenes as aryl reagents was developed. Azoles including oxazoles, thiazoles, imidazoles, 1,3,4-oxadiazoles, and oxazolines could react with 1-aryltriazenes smoothly to generate the corresponding products in good to excellent yields, and various substitution patterns were tolerated toward the reaction.

Total synthesis of micrococcin P1 and thiocillin I enabled by Mo(vi) catalyst

Thiopeptides are a class of potent antibiotics with promising therapeutic potential. We developed a novel Mo(vi)-oxide/picolinic acid catalyst for the cyclodehydration of cysteine peptides to form thiazoline heterocycles. With this powerful tool in hand, we completed the total syntheses of two representative thiopeptide antibiotics: micrococcin P1 and thiocillin I. These two concise syntheses (15 steps, longest linear sequence) feature a C-H activation strategy to install the trisubstituted pyridine core and thiazole groups. The synthetic material displays promising antimicrobial properties measured against a series of Gram-positive bacteria.

Cross-coupling of aromatic esters and amides

Catalytic cross-coupling reactions of aromatic esters and amides have recently gained considerable attention from synthetic chemists as de novo and efficient synthetic methods to form C-C and C-heteroatom bonds. Esters and amides can be used as diversifiable groups in metal-catalyzed cross-coupling: in a decarbonylative manner, they can be utilized as leaving groups, whereas in a non-decarbonylative manner, they can form ketone derivatives. In this review, recent advances of this research topic are discussed.

Post-translational modifications involved in the biosynthesis of thiopeptide antibiotics

Thiopeptide antibiotics are a class of typical ribosomally synthesized and post-translationally modified peptides (RiPPs) with complex chemical structures that are difficult to construct via chemical synthesis. To date, more than 100 thiopeptides have been discovered, and most of these compounds exhibit remarkable biological activities, such as antibacterial, antitumor and immunosuppressive activities. Therefore, studies of the biosynthesis of thiopeptides can contribute to the development of new drug leads and facilitate the understanding of the complex post-translational modifications (PTMs) of peptides and/or proteins. Since the biosynthetic gene clusters of thiopeptides were first discovered in 2009, several research studies regarding the biochemistry and enzymology of thiopeptide biosyntheses have been reported, indicating that their characteristic framework is constructed via a cascade of common PTMs and that additional specific PTMs diversify the molecules. In this review, we primarily summarize recent advances in understanding the biosynthesis of thiopeptide antibiotics and propose some potential applications based on our insights into the biosynthetic logic and machinery.

Chemoenzymatic synthesis of thiazolyl peptide natural products featuring an enzyme-catalyzed formal [4 + 2] cycloaddition

Thiocillins from Bacillus cereus ATCC 14579 are members of the well-known thiazolyl peptide class of natural product antibiotics, the biosynthesis of which has recently been shown to proceed via post-translational modification of ribosomally encoded precursor peptides. It has long been hypothesized that the final step of thiazolyl peptide biosynthesis involves a formal [4 + 2] cycloaddition between two dehydroalanines, a unique transformation that had eluded enzymatic characterization. Here we demonstrate that TclM, a single enzyme from the thiocillin biosynthetic pathway, catalyzes this transformation. To facilitate characterization of this new class of enzyme, we have developed a combined chemical and biological route to the complex peptide substrate, relying on chemical synthesis of a modified C-terminal fragment and coupling to a 38-residue leader peptide by means of native chemical ligation (NCL). This strategy, combined with active enzyme, provides a new chemoenzymatic route to this promising class of antibiotics.

Thiopeptide engineering: a multidisciplinary effort towards future drugs

The recent development of thiopeptide analogues of antibiotics has allowed some of the limitations inherent to these naturally occurring substances to be overcome. Chemical synthesis, semisynthetic derivatization, and engineering of the biosynthetic pathway have independently led to complementary modifications of various thiopeptides. Some of the new substances have displayed improved profiles, not only as antibiotics, but also as antiplasmodial and anticancer drugs. The design of novel molecules based on the thiopeptide scaffold appears to be the only strategy to exploit the high potential they have shown in vitro. Herein we present the most relevant achievements in the production of thiopeptide analogues and also discuss the way the different approaches might be combined in a multidisciplinary strategy to produce more sophisticated structures.

The endeavor of total synthesis and its impact on chemistry, biology and medicine

The synthesis of urea in 1828 set in motion the discipline of organic synthesis in general and of total synthesis in particular, the art and science of synthesizing natural products, the molecules of living nature. Early endeavors in total synthesis had as their main objective the proof of structure of the target molecule. Later on, the primary goal became the demonstration of the power of synthesis to construct complex molecules through appropriately devised strategies, making the endeavor an achievement whose value was measured by its elegance and efficiency. While these objectives continue to be important, contemporary endeavors in total synthesis are increasingly focused on practical aspects, including method development, efficiency, and biological and medical relevance. In this article, the emergence and evolution of total synthesis to its present state is traced, selected total syntheses from the author's laboratories are highlighted, and projections for the future of the field are discussed.

Thiopeptide antibiotics: retrospective and recent advances

Thiopeptides, or thiazolyl peptides, are a relatively new family of antibiotics that already counts with more than one hundred different entities. Although they are mainly isolated from soil bacteria, during the last decade, new members have been isolated from marine samples. Far from being limited to their innate antibacterial activity, thiopeptides have been found to possess a wide range of biological properties, including anticancer, antiplasmodial, immunosuppressive, etc. In spite of their ribosomal origin, these highly posttranslationally processed peptides have posed a fascinating synthetic challenge, prompting the development of various methodologies and strategies. Regardless of their limited solubility, intensive investigations are bringing thiopeptide derivatives closer to the clinic, where they are likely to show their veritable therapeutic potential.

Amythiamicin D and related thiopeptides as inhibitors of the bacterial elongation factor EF-Tu: modification of the amino acid at carbon atom C2 of ring C dramatically influences activity

Three analogues of amythiamicin D, which differ in the substitution pattern at the methine group adjacent to C2 of the thiazole ring C, were prepared by de novo total synthesis. In amythiamicin D, this carbon atom is (S)-isopropyl substituted. Two of the new analogues carry a hydroxymethyl in place of the isopropyl group, one at an S- (compound 3 a) and the other at an R-configured stereogenic center (3 b). The third analogue, 3 c, contains a benzyloxymethyl group at an S-configured stereogenic center. Compounds 3 b and 3 c showed no inhibitory effect toward various bacterial strains, nor did they influence the translation of firefly luciferase. In stark contrast, compound 3 a inhibited the growth of Gram-positive bacteria Staphylococcus aureus (strains NCTC and Mu50) and Listeria monocytogenes EGD. In the firefly luciferase assay it proved more potent than amythiamicin D, and rescue experiments provided evidence that translation inhibition is due to binding to the bacterial elongation factor Tu (EF-Tu). The results were rationalized by structural investigations and by molecular dynamics simulations of the free compounds in solution and bound to the EF-Tu binding site. The low affinity of compound 3 b was attributed to the absence of a critical hydrogen bond, which stabilizes the conformation required for binding to EF-Tu. Compound 3 c was shown not to comply with the binding properties of the binding site.

How thiostrepton was made in the laboratory

Thiostrepton, a powerful antibiotic belonging to the thiopeptide class, was synthesized in the laboratory for the first time in 2004 through an arduous campaign involving novel strategies and tactics, scenic detours, and unexpected roadblocks. In this Review the author narrates the long journey to success, not so dissimilar to Odysseus' return voyage to Ithaca, full of adventure, knowledge, and wisdom.

Constructing molecular complexity and diversity: total synthesis of natural products of biological and medicinal importance

The advent of organic synthesis and the understanding of the molecule as they occurred in the nineteenth century and were refined in the twentieth century constitute two of the most profound scientific developments of all time. These discoveries set in motion a revolution that shaped the landscape of the molecular sciences and changed the world. Organic synthesis played a major role in this revolution through its ability to construct the molecules of the living world and others like them whose primary element is carbon. Although the early beginnings of organic synthesis came about serendipitously, organic chemists quickly recognized its potential and moved decisively to advance and exploit it in myriad ways for the benefit of mankind. Indeed, from the early days of the synthesis of urea and the construction of the first carbon-carbon bond, the art of organic synthesis improved to impressively high levels of sophistication. Through its practice, today chemists can synthesize organic molecules--natural and designed--of all types of structural motifs and for all intents and purposes. The endeavor of constructing natural products--the organic molecules of nature--is justly called both a creative art and an exact science. Often called simply total synthesis, the replication of nature's molecules in the laboratory reflects and symbolizes the state of the art of synthesis in general. In the last few decades a surge in total synthesis endeavors around the world led to a remarkable collection of achievements that covers a wide ranging landscape of molecular complexity and diversity. In this article, we present highlights of some of our contributions in the field of total synthesis of natural products of biological and medicinal importance. For perspective, we also provide a listing of selected examples of additional natural products synthesized in other laboratories around the world over the last few years.

Nickel-catalyzed C-H arylation of azoles with haloarenes: scope, mechanism, and applications to the synthesis of bioactive molecules

Novel nickel-based catalytic systems for the C-H arylation of azoles with haloarenes and aryl triflates have been developed. We have established that Ni(OAc)(2)/bipy/LiOtBu serves as a general catalytic system for the coupling with aryl bromides and iodides as aryl electrophiles. For couplings with more challenging electrophiles, such as aryl chlorides and triflates, the Ni(OAc)(2)/dppf (dppf = 1,1'-bis(diphenylphosphino)ferrocene) system was found to be effective. Thiazoles, benzothiazoles, oxazoles, benzoxazoles, and benzimidazoles can be used as the heteroarene coupling partner. Upon further investigation, we discovered a new protocol for the present coupling using Mg(OtBu)(2) as a milder and less expensive alternative to LiOtBu. Attempts to reveal the mechanism of this nickel-catalyzed heterobiaryl coupling are also described. This newly developed methodology has been successfully applied to the syntheses of febuxostat (a xanthine oxidase inhibitor that is effective for the treatment of gout and hyperuricemia), tafamidis (effective for the treatment of TTR amyloid polyneuropathy), and texaline (a natural product having antitubercular activity).

Manipulation of thiocillin variants by prepeptide gene replacement: structure, conformation, and activity of heterocycle substitution mutants

Bacillus cereus ATCC 14579 converts the C-terminal 14 residues of a 52-mer prepeptide into a related set of eight variants of the thiocillin subclass of thiazolyl peptide antibiotics by a cascade of post-translational modifications that alter 13 of those 14 residues. We have introduced prepeptide gene variants into a knockout strain to conduct an alanine scan of all 14 progenitor residues, as well as a serine scan of the six cysteine residues that are converted to thiazoles in the mature natural product. No mature scaffolds were detected for the S1A and S10A mutants, consistent with their roles as the source of the pyridine core. In both the alanine and serine scans, only one substitution mutant failed to produce a mature scaffold: cysteine 11. Cysteine to serine mutants gave mixture of dehydrations, aromatizations, and unaltered alcohol side chains depending on position. Overall, substitutions that altered the trithiazolylpyridine core or reduced the conformational rigidity of the 26-membered macrocyclic loop led to loss of antibiotic activity. In total, 21 peptide mutants were cultured, from which production of 107 compounds was observed and 94 compounds, representing 17 structural mutants, were assayed for antibiotic activity. High-resolution NMR solution structures were determined for one mutant and one wild-type compound. These structures demonstrate that the tight conformational rigidity of the natural product is severely disrupted by loss of even a single heterocycle, perhaps accounting for the attendant loss of activity in such mutants.

Genetic interception and structural characterization of thiopeptide cyclization precursors from Bacillus cereus

The pyridine core of the thiocillins has long been postulated to arise from a late-stage tail-to-tail condensation of two dehydroalanines. Genetic disruption of tclM, a proposed "Diels-Alderase", allowed isolation of acyclic precursors to this pyridine ring. The isolated products possess the full cohort of post-translational modifications that are normally displayed by the thiocillins, including dehydrobutyrines, thiazoles, C-terminal decarboxylation, and the two previously unconfirmed dehydroalanines. Additionally, leader peptides have undergone extensive N-terminal degradation and the remaining leader peptide residues have been N-succinylated. These results identify TclM and its homologues in other thiazolyl peptide producing strains as the enzymes responsible for the trans-annular heteroannulation at core of this class of molecules.

Generation of thiocillin variants by prepeptide gene replacement and in vivo processing by Bacillus cereus

The thiocillins are natural-product antibiotics derived from ribosomally encoded peptides that undergo extensive posttranslational modifications to yield the mature trithiazolylpyridine-containing macrocyclic compound. Poor pharmacokinetic properties have prevented the clinical use of these highly potent antibiotics. Through in vivo manipulation of the gene responsible for production of the thiocillin precursor peptide, we have generated 65 novel thiocillin variants, allowing us to explore structure-activity relationships involved in both precursor peptide maturation and antibiotic activity.

Identification of distinct thiopeptide-antibiotic precursor lead compounds using translation machinery assays

Most thiopeptide antibiotics target the translational machinery: thiostrepton (ThS) and nosiheptide (NoS) target the ribosome and inhibit translation factor function, whereas GE2270A/T binds to the elongation factor EF-Tu and prevents ternary complex formation. We have used several in vitro translational machinery assays to screen a library of thiopeptide antibiotic precursor compounds and identified four families of precursor compounds that are either themselves inhibitory or are able to relieve the inhibitory effects of ThS, NoS, or GE2270T. Some of these precursors represent distinct compounds with respect to their ability to bind to ribosomes. The results not only provide insight into the mechanism of action of thiopeptide compounds but also demonstrate the potential of such assays for identifying lead compounds that might be missed using conventional inhibitory screening protocols.

Recent advances in the chemistry and biology of naturally occurring antibiotics

Ever since the world-shaping discovery of penicillin, nature's molecular diversity has been extensively screened for new medications and lead compounds in drug discovery. The search for agents intended to combat infectious diseases has been of particular interest and has enjoyed a high degree of success. Indeed, the history of antibiotics is marked with impressive discoveries and drug-development stories, the overwhelming majority of which have their origin in natural products. Chemistry, and in particular chemical synthesis, has played a major role in bringing naturally occurring antibiotics and their derivatives to the clinic, and no doubt these disciplines will continue to be key enabling technologies. In this review article, we highlight a number of recent discoveries and advances in the chemistry, biology, and medicine of naturally occurring antibiotics, with particular emphasis on total synthesis, analogue design, and biological evaluation of molecules with novel mechanisms of action.

Total synthesis of siomycin A: construction of synthetic segments

The five practical segments for the total synthesis of siomycin A, that is, the dehydropiperidine segment A (5), the pentapeptide segment B (3), the dihydroquinoline segment C (6), and the beta-phenylselenoalanine dipeptide segments D (7) and E (4), were synthesized. Segment A (5) was constructed by the coupling of the azomethine ylide and the chiral sulfinimine, followed by the stereoselective reduction of the six-membered imine function. Segment B (3) was synthesized by the phenylselenylation of the beta-lactone, stereoselective vinylzinc addition to the chiral sulfinimine, and oxazoline-thioamide conversion. Segment C (6) was prepared by the one-pot olefination of the tetrahydroquinoline N-oxide using triflic anhydride and triethylamine, stereoselective reduction of the methyl ketone function, and regioselective Yb(OTf)(3)-catalyzed epoxide opening by the amino group. Segments D (7) and E (4) were synthesized by coupling of the properly protected beta-phenylselenoalanines.

Total syntheses of amythiamicins A, B and C

Total syntheses of the thiopeptide antibiotics amythiamicins A, B and C are reported.

Concise total synthesis of the thiazolyl peptide antibiotic GE2270 A

The potent antibiotic thiazolylpeptide GE2270 A was synthesized starting from N-tert-butyloxycarbonyl protected valine in a longest linear sequence of 20 steps and with an overall yield of 4.8 %. Key strategy was the assembly of the 2,3,6-trisubstituted pyridine core by consecutive cross-coupling reactions starting from 2,6-dibromo-3-iodopyridine. The complete Southern fragment was installed by Negishi cross-coupling of 3-zincated 2,6-dibromopyridine at the terminal 2-iodothiazole of a trithiazole (87 %). The substituent at C-6 representing the Northern part of the molecule was introduced in form of the truncated tert-butyl 2-bromothiazole-4-carboxylate after metalation to a zinc reagent by another Negishi cross-coupling (48 %). Decisive step of the whole sequence was the macrocyclization to a 29-membered macrolactam, which was conducted as an intramolecular Stille cross-coupling occurring at C-2 of the pyridine core and providing the desired product in 75 % yield. The required stannane was obtained by amide bond formation (87 %) between a complex dithiazole fragment representing the Eastern part of GE2270 A and a 3,6-disubstituted 2-bromopyridine. Final steps included attachment of a serine-proline amide dipeptide to the Northern part of the molecule (65 %), formation of the oxazoline ring and silyl ether deprotection (55 % overall).

Total synthesis of thiopeptide antibiotics GE2270A, GE2270T, and GE2270C1

The total syntheses of the thiopeptide antibiotics GE2270A (7), GE2270T (8), and GE2270C1 (9) are described. The original synthetic strategies employed utilized the hetero-Diels-Alder reaction to construct the pyridine core of the target molecules and relied on a macrolactamization process to construct the macrocycle. The hetero-Diels-Alder-based strategy finally evolved allows the introduction of all four thiazole units attached to the pyridine ring and a one-pot sequence for macrocyclization and side-chain extension, culminating in highly convergent and expedient syntheses of these molecules as exemplified by a 24-step synthesis of GE2270C1 (9).

From Amino Acids to Heteroaromatics—Thiopeptide Antibiotics, Nature's Heterocyclic Peptides

Amino acids, the building blocks of proteins, also serve as precursors to a wide range of other naturally occurring substances including alkaloids, antibiotics, and, the subject of this Review, heterocyclic peptides. Simple alpha-amino acids are converted into complex arrays of heteroaromatic rings that display interesting and potent biological activity. The thiopeptide antibiotics, with their complex molecular architectures, are wonderful examples. In this Review we show how organic chemists have developed innovative methods for the synthesis of the heterocyclic ring systems, including routes inspired by the likely biosynthetic processes, and successfully assembled such building blocks into the final target molecule by application of orthogonal protecting groups and coupling methodologies.